The present invention relates generally to communication systems and to adapting a radio connection to the amount of data to be transmitted and to the prevailing radio traffic environment, and more particularly, to communication systems that can perform different types of interleaving depending on various conditions.
Second-generation cellular radio systems, such as D-AMPS (Digital Advanced Mobile Phone Service), GSM (Global System for Mobile telecommunications) and PDC (Personal Digital Cellular), are mainly meant for transmitting voice data using telephone connections that all have the same data transmission rate. The required data transmission rate can vary largely between the different connections, and it may even change during the communication. Moreover, the interference that invariably occurs in the radio connection may require coding on different levels in order to transmit the information carried in the connection to the receiver in a feasible form. Further, there has been an increasing demand for wireless networks to accommodate various types of traffic, such as voice, data, and video information, which requires various or increased data transmission rates.
There are presently two prevalent systems for wireless communications, TDMA and CDMA. In systems based on TDMA (Time Division Multiple Access), a given time slot is allocated for the use of each single connection, the time slot being part of a cyclically repeated frame structure at a given carrier wave frequency. In systems based on CDMA (Code Division Multiple Access), the corresponding basic unit allocated for the use of a connection is a given division code within a given carrier wave frequency. Current wireless systems are limited to data speeds around 1 Mbits/sec. The main limitation is inter-symbol interference due to multipath and frequency selective fading. OFDM (Orthognal Frequency Division Multiplexing) overcomes this limitation. In OFDM, a high-speed serial data stream is converted to many low speed parallel streams. The low speed parallel streams are then transmitted simultaneously using many subcarriers. Since the data rate of each parallel stream is much lower, the bandwidth of each subcarrier is generally smaller than the coherent bandwidth, which can eliminate inter-symbol interference caused by linear distortion. In addition, the spectra of the OFDM subcarriers overlap each other with minimum separation. Thus, OFDM can achieve high speed and high spectrum efficiency.
A version of CDMA based on OFDM, for use in fourth-generation cell phones, known as multicarrier CDMA (MC-CDMA), is currently being developed. MC-CDMA transfers a given symbol over multiple orthogonal subcarriers. Yet another version of CDMA based on OFDM is known as multicarrier/direct spread (MC/DS) CDMA, which uses multiple subcarriers, which have been directly spread processed.
The goal of any communication system is the error free transmission of the communicated signal whether it be an analog signal, a coded analog signal, or coded data. Communication systems adapted for transmission of coded information (voice and/or data) generally include some form of error correction. Error correction in these systems frequently takes the form of error detection and correction software. That is, software adapted to detect errors in the coded information and based upon a set of correction parameters, replace the errors with an estimation of the correct coded information. These and other types of error correction mechanisms typically rely on prediction, interpolation and other similar techniques that generate an estimation of the corrupted coded information from preceding and succeeding bits of coded information.
In wireless communication systems, such as the types discussed above, bursty errors, due, for example, to fading, interference or other disruptions to the coded information as it is transmitted over the air interface, may cause errors in blocks of bits. Errors in blocks of bits are difficult to correct using error correcting code because of the lack of surrounding information from which to estimate the correct information. A solution is to provide as much diversity in the air interface as possible to achieve acceptable levels of communication quality in terms of data error rate. A common technique for introducing diversity into the air interface is interleaving the bits of coded information over many transmitted frames. Interleaving, in addition to error correction, works very well especially where fading is experienced by scattering the errors that would otherwise wipe out an entire frame of coded information among many frames.
Various kinds of interleaving are known, such as chip interleaving, bit interleaving and symbol interleaving. In chip interleaving, a symbol (bit of coded information) is distributed to non-adjacent subcarriers. Chip interleaving is performed after spreading in the transmitter and chip de-interleaving is performed before de-spreading in the receiver. In bit interleaving, the constituent bits of a modulated symbol are mapped onto different subcarriers instead of the same subcarrier. Bit interleaving is used to deter the attenuation of consecutive bits by randomizing the bit sequence before modulation. In symbol interleaving, symbols of an RF transmit frame are read into a matrix one way and read out for transmission in a different way. For example, the data is read into the matrix by rows and read out of the matrix by columns. The next frame is then read into the matrix and the process repeated. Symbol interleaving maintains the correlation between subcarriers within a symbol. This is the method specified by the IS-95 standard where 456 symbols of a 20 millisecond (ms) frame are interleaved prior to spreading of the signal.
For MC-CDMA, symbol interleaving can maintain orthogonality using correlation between adjacent subcarriers in a multipath environment with short delay spread. On the other hand, chip and bit interleaving can obtain high frequency diversity gain by distributing a symbol over an uncorrelated subcarrier. The optimum interleaving method depends on the level of inter-code interference, which depends upon the number of active codes. The number of active codes generally is related to the number of users. For instance, if each user uses only one code at a time, then the number of active codes is equal to the number of users. More particularly, symbol interleaving is advantageous when there are many active codes, while chip interleaving and bit interleaving are advantageous when there are fewer active codes.
It would be advantageous to be able to dynamically select a desirable interleaving method based on various network factors.